You can't put handguards on because with handguards, I'm pretty sure you'll exceed the 50 oz. weight limit for a post ban assault pistol. You could attach a forward grip only if you first register the gun as an "Any Other Weapon" (AOW) on a form 1 or have a manufacturer do it for you on a form 1 and then transfer it to you on a form 4.

A handguard or heatshield of any kind would make it an illegal "assault weapon." This was put in specifically to ban the TEC-9 and variants.
A front pistol grip would turn the gun into an NFA-controlled "Any Other Weapon", or AOW.
-Troy